1. Field of the Invention
The invention relates to a method for operating a brake system of the type having electrically triggerable hydraulic valves and particularly suited for use in motor vehicles.
2. Description of the Prior Art
Brake systems known in the industry, besides comfort-oriented functions, also have safety-relevant operating strategies, which are made available in so-called ABS systems or ESP units. Particularly in braking interventions on the part of the brake system that stabilize a vehicle, fast reaction times are necessary once a safety-critical driving situation is detected, and such times are a function of the particular pressure buildup dynamics of a brake system.
At very low ambient temperatures of a vehicle, because of increased viscosity of the brake fluid of a brake system, the pressure buildup dynamics of a brake system is impaired so severely that sufficiently fast reaction times of a brake system can be attained only with difficulty, since high pressure losses occur as a result of the increased viscosity.
From U.S. Pat. No. 5,971,503, a hydraulic control system with a hydraulic pump and with a device that is actuatable as a function of a hydraulic pressure is known. The control system includes hydraulic lines, hydraulic valves, and an electronic control circuit for supplying current to the hydraulic valves, in order, by supplying current in the region of the hydraulic valves, to warm a hydraulic fluid located in the brake system if this fluid has a viscosity that impairs the pressure buildup dynamics.
However, it is disadvantageous that the hydraulic valves are supplied with current in such a way that during a warming phase of the hydraulic fluid of the brake system, they remain in a mode of operation that is equivalent to a currentless switching state, and the warming of the hydraulic fluid is accomplished with only a slight heating output, so that the pressure buildup dynamics of the brake system is not improved until after an undesirably long period of time.
In the method known from German Patent Disclosure DE 101 63 524 A1 for triggering a braking system for motor vehicles with electrically triggerable hydraulic valves and a hydraulic pump, one or more existing valve coils, required for hydraulic control functions, are triggered for the sake of electrical heating of one or more holders that hold these coils; the triggering is done for longer lengths of time than necessary for the regulating function and/or in periods of time in which it is unnecessary for the regulating functions, and the triggering is also done with current intensities that are suitable for opening the respective valve when it is closed when without current, or to close it when it is open when without current.
In this method, the hydraulic valves are supplied with higher current intensities than in the method known from U.S. Pat. No. 5,971,503. Thus the hydraulic fluid of the braking system is warmed faster in the direction of a temperature at which the hydraulic fluid has a viscosity that makes improved pressure buildup dynamics of a brake system possible.
It is provided in this reference that the hydraulic fluid of the braking system is first warmed during a first heating phase, which includes a first heating pulse, followed by an interval without heating and at least one further heating pulse; after the first heating phase, at least one temperature measurement is performed, and particularly during the heating phases, no temperature measurement is performed. It is furthermore provided that second or further heating phases are not started until, or are started only whenever, the temperature measured by a temperature sensor is below a predetermined threshold temperature.
However, it is disadvantageous here that direct measurement of the temperature of the hydraulic fluid of the brake system in the region of hydraulic valves of a brake system requires great effort and expense for equipment and therefore entails high costs.
To reduce the costs for the temperature determination of the hydraulic fluid, it is possible for instance, by measurement during operation, to determine a magnitude for warming the hydraulic fluid of the brake system, this magnitude being equivalent to the temperature of the hydraulic fluid in the region of the hydraulic valves that are being supplied with current. For instance, it is possible to use an existing temperature sensor in the brake system that is positioned as close as possible to the hydraulic block of the braking system.
However, this kind of temperature determination is disadvantageously affected by a heat development, occurring during operation of the braking system, in the region of the electric control unit of the braking system. This is due to the fact that because of the temperature sensor already present in the brake system, under some circumstances the temperature increase, which occurs in operation in the region of the control unit and does not cause significant warming of the hydraulic fluid, affects the temperature ascertainment to such an extent that because of incorrect measurement and since the temperature ascertained via the temperature sensor is far from being equivalent to the temperature of the hydraulic fluid, the hydraulic fluid is not warmed as needed to improve the pressure buildup dynamics of the braking system, and the heating of the brake system is ended even though the hydraulic fluid is still, as before, at a temperature that impairs the pressure buildup dynamics of the brake system.
It is therefore the object of the present invention to make a method available for operating a brake system for motor vehicles with electrically triggerable hydraulic valves by means of which method the pressure buildup dynamics of a brake system can be improved in a simple, economical way.